Difference between 511A and 511AX

Got my first taste on a grinder this weekend, an old Nielson unit. I've been looking at the Northern Grinder as well as the Bailey's models since. It seems the Oregon 511 models seem to be the benchmark in that class of personal use grinders. There are a few 511A models on ebay under $300 compared to the 511AX models that are closer to $400. What is the difference? Moreso, what is the functional difference between the Oregon grinders compared to the Northern Tool Grinder with the shim upgrades?

Check out this thread. The Tecomec Jolly Star which is the made by the manufacturer of Oregon's Grinders. They were delayed while they made an improvement and added rollers to the rails so the chain would slide easier. These grinders also go from the smallest chain all the way to harvester chain. Go download the brochure on the thread below.